Air cathodes generally consist of three major components. The first component consists of a thin electrolyte-permeable layer facing the electrolyte side of the cell. The second component consists of one or more active layers containing activated carbon and optionally a catalyst, these active layers being located in an intermediate portion of the electrode where the electrochemical reaction takes place. The third component is a wet-proofing layer that faces the gas side of the cell. The whole electrode structure is supported on the first layer which also acts as a current carrier. In the case of a current-producing cell, the current carrier is called a "current collector" and in the case of an electrolytic cell, the current carrier is called a "current distributor".
In the operation of an air cathode, oxygen from the air is reduced to hydroxide ion (OH.sup.-) according to the following reaction: EQU O.sub.2 +H.sub.2 O+2e.revreaction.HO.sub.2.sup.- +OH.sup.-
Active carbon used in the intermediate layer of the electrode acts as a catalyst for the oxygen reduction to hydroxide ion; the reaction also produces peroxide ion (HO.sub.2.sup.-) as an intermediate product. The peroxide ion is then decomposed to produce additional hydroxide (final product). The decomposition reaction may be represented as follows: EQU HO.sub.2.sup.-.fwdarw.OH.sup.31 +1/2O.sub.2
Various catalysts are known for this reaction such as platinum, gold, silver, spinel, etc.
It is important in operation of an air cathode to decompose the peroxide ion as soon as it is formed in the active carbon layer. The peroxide ion, if allowed to accumulate in the active layer in any appreciable amounts, will attack and oxidize the carbon and also give rise to excessive concentration polarization. In order to help alleviate this problem, it has become the practice to include in the active layer a catalyst which will promote the decomposition reaction. The problem with this approach is that the reaction does not proceed at a fast enough rate to decompose all of the HO.sub.2.sup.- without using large quantities of the catalyst, which is expensive and impractical. Consequently, there is a tendency for the peroxide to accumulate along with other reaction products in the active layer of the electrode. This excessive accumulation of HO.sub.2.sup.- interferes with optimum cell operating leading to decreased half cell potential for any given current density and results in decreased electrode life.
It is therefore important to provide for effective mass transport of reaction products from the electrode structure in order to minimize the effects of concentration polarization.
Air cathodes have heretofore employed many different types of metal current collectors. Probably the most common type of current collector has been a wire mesh screen or a similar structure. Electrodes can be easily fabricated using a wire mesh screen simply by pressing the active material onto one side of the screen. The wire mesh screen serves as a conductive member for transporting electrons and also acts as a mechanical support for the electrode structure.
A major problem with the wire mesh screen, however, has been that, due to its relatively smooth surface, the screen does not bond very well with the active carbon layer. The active layer may at times separate from the current collector, particularly if the electrode has been subjected to severe or abusive operating conditions.
U.S. Pat. No. 3,556,856 discloses a three-layer electrode structure which effectively avoids this problem. In this electrode structure, one layer containing the active material is coherently bonded to two other layers that together serve as the current collector. This current collector comprises a first layer of porous metal containing relatively fine pores and a second layer containing coarse metal particles. The first layer may be made from a thin sintered metal plaque composed of fine metal particles, for example, and serves to support the electrode. The second layer, containing coarse metal particles, provides an excellent interface to achieve a strong cohesive bond with the active carbon layer. Although this so-called "biporous" metal current collector offers the advantage of superior electrode strength, the relatively fine pore structure in the support layer can give rise to serious mass transport problems for the reaction products.
It is therefore an important object of the present invention to provide an improved metal current carrier for use in fabricating electrochemical cell electrodes.
Another object is to provide such an improved metal current carrier which will have increased mass transport properties to prevent concentration build-up of reaction products within the electrode.
Still another object of the present invention is to provide an electrochemical cell electrode utilizing such an improved metal current carrier in its fabrication.
Still another object of the present invention is to provide an air cathode of this construction which will significantly increase catalytic activity for decomposing peroxide ions produced in the cell reaction.
Still another object of the present invention is to provide such an air cathode in which intimate, coherent, well-bonded contact between the current carrier and active carbon layer can be achieved.
A further object of the present invention is to provide a novel method for fabricating the improved metal current carrier and air cell electrode.